Precision diffraction gratings have been manufactured since about 1882, and have been attributed to H. A. Rowland of Johns Hopkins University. A precision ruling engine is used to scribe very fine parallel lines in a glass or silica substrate. The methodology has been well understood for decades. There are advantages to using diffraction gratings in reflection applications, but for the case of low reflectivity substrate materials, such as silica, efficient gratings require a highly reflecting film to be vacuum deposited over the rulings. Aluminum is the preferred metal for vacuum deposition over the ruled substrate. Alternatively, aluminum can be vacuum deposited onto a glass or silica substrate. The rulings can then be cut directly into the aluminum film. However, freshly deposited aluminum films quickly form an oxide layer at the surface. The oxide layer is slightly absorbing for ultraviolet radiation, especially for wavelengths shorter than 200 nm. Hence, for ultraviolet application, a thin protective overcoat layer, typically of MgF2, has been immediately deposited over the aluminum film as a means of preventing the aluminum from oxidation. The optimum layer thicknesses, the deposition techniques, and the process parameters for aluminum deposition, and for the protective overcoat depositions, are well known to those skilled in the art (see G. Hass, J. Opt. Soc. Am. Vol. 39 (1949), p. 179; R. Madden, Physics of Thin Films, Vol. 1. (Academic Press, New York 1963); and Canfield et al., Applied Optics, Vol. 14 (1975), pp. 2639-44). Techniques for depositing additional dielectric films to enhance the reflectance of aluminum are also well known and in common use (see J. Hass, ibid. and J. Phys. Radium Vol. 11 (1950), p. 394).
High power excimer lasers such as KrF, ArF, and F2 lasers that operate at 248 nm, 193 nm and 157 nm, respectively, are light sources of choice for microlithographic applications. However, while output of such lasers is theoretically at a single wavelength, in fact the laser's output is not sufficiently monochromatic and must be spectrally reduced or “narrowed”. Diffraction gratings, among other components, are key elements for narrowing the spectrum emitted by the laser. The grating achieves the narrowing by reflecting back into the laser's resonating cavity only a narrow range of wavelengths. Light energy at this narrow wavelength range resonates within the cavity and is emitted through a partially reflective mirror at the other end of the cavity. Typically, a master grating is first manufactured and then the master is replicated to form additional gratings. Each of the replicated gratings may then be used as a master grating to form additional replicas. The diffraction grating, be it a master grating or a replication of a master, must be highly reflective. High reflectivity is generally accomplished using an aluminum substrate and/or a high quality film deposition of aluminum onto a grating substrate.
The current state of the art of grating manufacture, including methods for obtaining enhanced reflectivity using thin film depositions, is described in U.S. Pat. No. 5,999,318 (the '318 patent), U.S. Pat. No. 6,511,703 (the “703 patent) and U.S. Pat. No. 6,529,321 the “321 patent). While the grating and methods for producing them described in the foregoing patents have proved useful to date, the industry has been increasing the average power delivered by excimer lasers. For example, the use of high power lasers with peak energy density (fluence) of >50 mJ/cm2 with pulses lengths in the 10 nanosecond (“ns”) range, pulse rates have increased by factors of 10 to 2 KHz and 4 KHz, and operating at wavelengths below 250 nm is becoming common. As a result of the use of such high power lasers, the lifetime of the laser elements (as measured by pulse count) such as mirrors and diffraction gratings has deteriorated. As a result, laser lithographic system operating time has been reduced.
Gratings are perhaps the most vulnerable components of the excimer laser system. Grating failures cause shutdowns of the lithography tool and replacement of the entire grating-containing module. Since lithography tools are very expensive, costing in the range of approximately $3MM to $10MM, the owners of such tools expect them to run twenty-fours a day, seven days a week. Unexpected and/or frequent shut downs for maintenance are very costly and disruptive to their production. While the replacement module containing the grating is expensive, the cost of the module is small in comparison to the cost of lost production resulting from failure of the tool. As a result, extended lifetimes for laser components such as diffraction gratings and mirrors are essential to both the microlithographic industry and to the manufacturers of the excimer laser systems used by the industry. Grating failure is generally caused by low reflectivity of the aluminum coating; which in turn is caused by oxide formation on the aluminum coating. In view of the system failures that can occur when gratings as presently known are used in high power excimer laser systems, and the high costs associated with such failures, there is a need for improved gratings with extended lifetime that can be used in such high power excimer lasers. The present invention presents a solution to this problem by providing improved gratings with extended lifetimes.